This invention relates to thermistor elements which are used for detection of temperature and temperature compensation of circuits. In particular, this invention relates to thermistor elements having an external electrode structure which is suited for surface mounting.
Since high-density mounting of electronic components is desired, thermistor elements are required to be surface-mountable, say, to a printed circuit board. FIGS. 13 and 14 (or FIGS. 13A, 13B, 14A and 14B) show examples of prior art surface-mountable thermistor elements.
FIGS. 13A and 13B show a thermistor element 61 having electrodes 63 and 64 formed so as to cover the two end surfaces of a thermistor body 62 made of a material with resistance having a negative temperature coefficient (NTC). These electrodes 63 and 64 are each formed not only on one of the end surfaces of the thermistor body 62 in the shape of a rectangular parallelepiped but also so as to reach the remaining four surfaces adjoining that end surface, that is, the upper, lower and two side surfaces. Thus, such a thermistor element 61 could easily be surface-mounted by attaching its lower surface 62a to an electrode land formed on a printed circuit board, for example, by soldering.
FIGS. 14A and 14B show a thermistor element 65 of the type disclosed in Japanese Patent Publication Tokkai 7-29704, characterized as having a first electrode 67 and a second electrode 68 formed on the lower surface of a thermistor body 66 in the shape of a rectangular parallelepiped so as to be mutually opposite to each other with a specified distance therebetween. If such a thermistor element 65 is desired to be miniaturized and the distance between its electrodes 67 and 68 is excessively reduced, however, there arises the danger of a short-circuiting.
In order to prevent the occurrence of short-circuiting, the thermistor element 65 is provided with an insulating layer 71 of an inorganic material, as shown in FIGS. 14A and 14B, so as to cover the lower surface of the thermistor body 66 between two external electrodes 69 and 70 which are formed respectively on the first and second electrodes 67 and 68, separated from each other by a distance larger than the gap between the first electrode 67 and the second electrode 68. Since the first and second electrodes 67 and 68, as well as these external electrodes 69 and 70, are all formed only on the lower surface of the thermistor body 66 without reaching any other surfaces, the thermistor element 65, too, can be easily surface-mounted by attaching its lower surface 66a to a printed circuit board, for example, by using a solder for reflow mounting or flow mounting.
With the thermistor element 61 shown in FIGS. 13A and 13B, each of the electrodes 63 and 64 is formed so as to reach five of the surfaces of the thermistor body 62. Thus, although it can be surface-mounted, say, onto a printed circuit board by soldering, the solder tends to form swollen parts referred to as xe2x80x9cfilletsxe2x80x9d which make high-density mounting difficult. This may be explained as follows. Suppose that the thermistor element 62 is surface-mounted onto a printed circuit board by applying a solder on the lower surface 62a of the thermistor body 62. If this is done, the parts of the electrodes 63 and 64 situated on the lower surface of the thermistor body 62 may be joined by the solder but the molten solder will swell along the three surfaces perpendicular to the lower surface of the thermistor body 62 and fillets are thereby formed. Thus, the area required for the mounting becomes far greater than the flat area of the thermistor element 61. This is a serious problem in the attempt to achieve high-density mounting.
As for the thermistor element 65 shown in FIGS. 14A and 14B, on the other hand, the external electrodes 69 and 70 for making connections are provided only on the lower surface of the thermistor body 66. Thus, there is no problem of fillets and hence the area for mounting can be made smaller and higher-density mounting can be accomplished than in the case of the thermistor element 61 of FIGS. 13A and 13B. The thermistor element 65 of FIGS. 14A and 14B, however, was originally for using a reflow mounting method with a solder paste or a flow mounting method with molten solder. Thus, higher mounting densities are very difficult to achieve with such mounting methods, for example, for the following reasons:
(1) High density mounting is not possible unless solder lands to be formed (say, on a printed circuit board) by a printing process is done with a high degree of accuracy but there have been limits to the accuracy in the printing of solder lands;
(2) When a solder material is melted, the thermistor element tends to be displaced from the solder land onto the base board; and
(3) It is difficult to control the thickness of a solder layer and hence it was difficult to control the mounting displacement of the thermistor element in the direction of the height.
By the reflow and flow methods, furthermore, the mechanical strength of joint becomes weaker due to the embrittlement of the solder and the electrical connections of the chip parts are sometimes deteriorated. Since thermistors which are used for the detection of temperature are required to be accurate to the level of about 1%, such a deterioration of electrical contacts could be a fatal defect.
Recently, a new mounting method referred to as the bump mounting is becoming popular as an improved method of mounting by which higher density mounting becomes possible than by the reflow or flow mounting method. The bump mounting method is a technology whereby a cylindrical or square pillared protrusion called a bump, usually comprising Au or Snxe2x80x94Pb, is inserted between a chip component and a base board and the bump is joined together with the board and the chip component by thermocompression bonding or by eutectic alloy formation.
By this method, a bump can be formed on a chip component or a base board with very high accuracy and, as long as a bump can be formed accurately, the chip component can be accurately attached to the base board. Another advantage of this method is that there is no problem of fillets.
Among the bump joints, Au bump joints are particularly favorable because they have a high mechanical strength and hence there is no embrittlement problem of the kind encountered with solder materials. Thus, reliable joints can be thereby realized.
The prior art thermistor elements 61 and 65 described above, however, are not suited for bump mounting because they were basically intended to be mounted by using a solder material, having the base layers for their electrodes comprised of a conductive paste. In other words, the electrodes 63 and 64 are formed by applying a conductive paste on a thermistor body 62 and baking it in order to obtain base layers and then forming a layer of Sn or a Snxe2x80x94Pb alloy in order to improve the solder wettability. As for the thermistor element 65, its first and second electrodes 67 and 68 are formed by applying a conductive paste such as of Ag on the lower surface 66a of the thermistor body 66 and then subjecting them to a baking process.
Thus, if external electrode layers for external connections are formed by plating Ni or Snxe2x80x94Pb on the electrodes formed by applying a conductive paste and subjecting it to a baking process as described above, the base layers are thick and uneven. As a result, the surfaces of the external electrodes thereabove were necessarily also uneven.
If a thermistor element is to be mounted onto a base board by a bump mounting method, the bumps and the electrodes of the thermistor element must be firmly in contact with each other. Thus, if the thermistor has external electrodes with very uneven surfaces with large indentations and protrusions, a dependably firm contact cannot be expected by a bump joint method.
It is therefore an object of this invention to provide thermistor elements suitable for surface mounting by bump joints, having reliable connections.
A thermistor element according to this invention, with which the above and other objects can be accomplished, may be characterized not only as comprising a thermistor body and a pair of electrodes formed mutually opposite to each other on one of the surfaces of the thermistor body, but also wherein these electrodes are in ohmic contact with the thermistor body and each comprise a thin-film contact layer and an external electrode layer which is formed either directly or indirectly over the contact layer and only on the surface on which the pair of electrodes is formed opposite each other and is of a metallic material such as Au, Ag, Pd, Pt, Sn and their alloys. Since the contact layer of each electrode is formed by a thin-film forming method, its surface is much smoother than the surfaces of prior art thick-film electrode layers formed by applying a conductive paste and subjecting to a firing process. As a result, the external electrode layer which is formed thereover also has a much smoother surface than those of prior art thermistor elements. Thus, when a thermistor element according to this invention is mounted by a bump bonding method, there is an improved reliability in the connection between the bumps and the external electrode layer. Thermistor elements according to this invention, however, may be mounted by a flow or reflow method using a solder. In other words, the invention is not limited by the method of mounting the thermistor elements.
According to a bump mounting method, bumps are inserted between the external electrode layers of the thermistor element and a circuit board and heat is applied to connect the bumps to the wires or lead terminals on the circuit board as well as to the external electrode layers of the thermistor element such that the thermistor element are connected both mechanically and electrically to the mounting board.
Au, Au alloys and Snxe2x80x94Pb alloys are commonly used for bumps. The external electrode layers are of a material such as Au, Ag, Pd, Pt, Sn and their alloys, according to the kind of the bump material such that the reliability of connection by the bump bonding can be even more improved. If the bumps comprise Au or a Au alloy, the external electrode layers are preferably formed with Au or a Au alloy. In other words, the reliability of bonding between the bumps and the external electrode layer can be improved if they both contain a same metal.
According to a preferred embodiment of this invention, the contact layers of the pair of electrodes are formed only on one surface of the thermistor body on which the pair of electrodes is formed opposite each other. Although the contact layers are not prevented from extending over to other surfaces of the thermistor body, formation of fillets can be reliably prevented when a solder flow or reflow method is used if the contact layers are only on the surface of the thermistor body on which the pair of mutually opposite electrodes is formed.
The contact layers are preferably of a metallic material such as Ni, Cr, Cu, Au, Ag and their alloys capable of reliably forming an ohmic contact with the thermistor body. The desired characteristics of the thermistor element can thus be dependably delivered through its electrodes.
The external electrode layers may be formed either directly over the contact layers or indirectly with an intermediate layer or two in between. There may be a single intermediate layer of a material such as Ni, Cu and their alloys, or there may be a second intermediate layer of a material such as Au, Ag, Pd, Pt, Sn and their alloys between the first intermediate layer and the contact layer. An intermediate layer of Ni, Cu or their alloy serves to form an alloy with a solder even if the external electrode layer is invaded such that a sufficiently strong bonding can be preserved and hence the thermistor element can be mounted also by a solder flow or reflow method. The second intermediate layer as described above serves to improve mechanical connections between contact layers and external layers.
It is preferable to also provide an insulative resin layer which will cover at least a portion of the same surface of the thermistor body on which the electrodes are formed opposite each other. Such an insulative resin layer serves to improve the resistance of the thermistor element against moisture and to prevent attachment of solder bridges when the thermistor element is mounted by a solder reflow or flow method, reducing the possibility of shorting between the electrodes even if the distance of their separation is relatively small. Such an insulative resin layer may be formed so as to cover portions of the electrodes such as their edge areas which are opposite each other or to extend over to surfaces other than the one on which the electrodes are formed.
There may be provided a second insulative resin layer on the surface of the thermistor body opposite to the one on which the electrodes are formed. The resistance of the thermistor element against moisture can be further improved with two surfaces of the thermistor body thus covered with insulative resin layers.
The pair of electrodes is not limited to be formed only on one surface. They may be formed opposite each other on different surfaces of the thermistor body.